Spontaneous unicompartment knee prosthesis tibial insert dislodgement
Sudden severe left knee pain. Previous unicompartment (medial) knee prosthesis insertion
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Two linear radio-opaque markers outline the polyethylene insert that normally sits on the tibial component of the unicompartment (medial) knee prosthesis.
In 2012 the markers are symmetrically positioned between the femoral and tibial components with an associated lucent gap - this is normal.
On the later examination the markers are antero-medial to the articulation with loss of the lucent articular gap indicating insert dislodgement.
Polyethylene spacers or inserts are inserted between the 2 components of a prosthesis for articulation much like articular cartilage.
They are attached to the tibial component in a knee prosthesis and are prone to wear over time and can lead to foreign body reaction (particle disease or aggressive granulomatosis). Rarely they can dislodge.
The polyethylene is radiolucent thus radio-opaque markers are inserted by the manufacturer so that their position can be monitored.